New Study: Forest Growth at Risk Despite Rising CO2 Levels

Rising CO2 levels will spur the growth of forests, which store carbon, but an experiment suggests this effect could be restricted by the availability of phosphorus in the soil

Thu 06 Jun 24


Soil microbes can now outcompete plants for vital nutrients, limiting forest growth and the amount of carbon dioxide forests can absorb from the atmosphere. This, in turn, can impact forests’ capacity to sequester carbon, which is vital to fighting climate change.

That is according to new research, Microbial competition for phosphorus limits the CO2 response of a mature forest, published in Nature Journal, which collected six years of data in mature New South Wales forests as part of the Eucalyptus Free Air CO2 Enrichment (EucFACE) experiment.

The world’s only Free-Air Carbon Dioxide Enrichment experiment in a mature, warm temperate forest ecosystem, EucFACE, is designed to predict the effects of rapidly rising atmospheric carbon dioxide on Australia’s unique native forests.

According to Dr Kristine Crous, Senior Lecturer at Western Sydney University’s Hawkesbury Institute for the Environment, higher CO2 levels usually increase plant growth by enhancing photosynthesis. However, this can only go so far with growth limited by nutrients available in the soil – in about one-third to half of all ecosystems; phosphorus is the nutrient that limits growth.

Until now, researchers remain uncertain about where those phosphorus limits are. One key unknown is how the amount of available phosphorus might change as plants and soil microorganisms respond to rising levels of CO2.

This led Dr Crous to lead a team of global scientists to collect data on changing phosphorus levels in Australian forests. Plots were exposed to artificially increased levels of CO2 using pipes hanging around the trees.

The team found that the amount of available phosphorus stayed the same with added CO2 despite the plants releasing more carbon into the soil through their roots. According to Dr Peter Reich, Director of the Institute for Global Change Biology at the University of Michigan’s School for Environment and Sustainability, some had thought this would spur soil microbes to recycle more phosphorus from dead and decaying matter.

The researchers attribute this to the microbes outcompeting the plants for any available phosphorus: the microbes contained more than triple the amount of phosphorus held within the plants. If this microbe-driven phosphorus limit is widespread, forests might respond less than expected to CO2 fertilisation, Dr Crous said. “Most models do not take the effects of low phosphorus into account and therefore overestimate ecosystem productivity.”

Nutrients may need to be added to some ecosystems to allow them to reach their full carbon storage potential, she says. However, it is an open question how much these results apply to forests elsewhere, according to César Terrer, an Assistant Professor of Civil Engineering at the Massachusetts Institute of Technology. And nutrients are just part of the picture. Increased drought, heat and fires associated with climate change are changing carbon storage in forests more than their direct response to CO2, Dr Terrer said.


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